Pulp refining and bleaching process



, 30 and more Patented Sept. 27, 1932 UNITED STATES GEORGE A. RI HTER, F BERLIN, NEW nam BERLIN, NEw HAMPSHIRE,

PULP REFINING AND BLEAcHING rEooE'ss -No Drawing. Application filed Kay 6,

This invention relates to the refining and bleaching of cellulose pulp, and has for its object the production of a product bleached substantially to ultimate whiteness and possessing certain optimum physical and chemical characteristics.

The usual bleachingof cellulose'pulp effects both physical andchemical injury uponthe fiber, the extent of such injury being determinable by physical characteristics such as tear resistance, and chemical characteristics such as copper number. Such in'ury may be materially reduced if the pulp is subjected to a distinctly alkaline bleaching treatment,

as disclosed in application, Serial No. 716,153,

filed May 27, 1924:, by Milton 0. Schur and myself, but even in such latter case the fiber is sometimes injured, particularly when the pulpis of a refractory nature and hence necessltates a relatively large amount of bleach to reach substantially ultimate whiteness. I

have now found that injury to, the fiber. may

be minimized, if in lieu of bleaching pulp to completion by one distinctly alkalinebleachingtreatment, such pulp is subjected to a series of such treatments, preferably with alternating washings.

. Such bleaching procedure finds especial utility in a process of refining raw wood pulp,

particularly alkaline-digested pulps such as kraft and soda pulps, so called,

and intended for the production of very white fiber of high alpha cellulose content and possessing characteristics which render it emi-- nently suitable as a raw material for those manufactures where a substantially pure form of cellulose containing a very low percentage of objectionable impurities such as less-resistant celluloses, oxycelluloses, pentosans, lignin, and resin, is desired. Such fiber may be used as a substitute for cotton fiber in paper manufacture and in the preparation of cellulose derivatives such as nitrocellulose, cellulose acetate and cellulose xanthate, or cel- 1927. Serial No. 7 189,461.

lulose manufactures such as rayon, lacquers, celluloid, and the like.

Her'etofore it has been difficult to refine an alkaline-digestedpulp, such as kraft, to a substantially pure form of cellulose, owing to the high percentage of pentosans, lignin, and other coloring materials which are present in such pulp. Moreover, certain of the wood components are apparently transformed by alkaline digestion into dyestufis which become so'tenaciously fixed on the fiber that they are difiicultto remove without degrad-' ing or injuring the fiber. It has been proposed to refine kraft pulp by first treating it with chlorine water, then redigesting it under pressure and at elevated temperature in 0 an alkaline solution, and finally bleaching it. With'such a process, however, if suflicient chlorine be used in the first step or sufiicient bleach in the final step to produce fiber of high whiteness, it is found that such fiber has been permanently injured from both a physical and chemical standpoint.

Inmy application, Serial-No. 73,193, filed December/l, 1925, 1 have described a process of refining alkaline-digested pulp. In accordance with that process, alkaline redigestion of the pulp is effected in a relatively concentrated alkaline solution at relatively low temperatures, for under such conditions it was found that the non-alpha cellulose components of such pulp, particularly the pentosans, were more effectively removed than in a-relatively, dilute solution at ele vated temperatures. By first treating the- 0 pulp with chlorine water or hypochlorite, then redigesting such pulp in a relatively concentrated alkaline solution at relatively low temperatures, it was found possible to bleach the redigested pulp to a whiteness approximating that of ordinary bleached pulp, without material injury to the fiber, if a thoroughly cooked pulp is employed as a raw material. The treatments before and after alkaline redigestion, however, must be carefully controlled, for if either is overdone in order to obtain the desired whiteness of fiber,-this being articularly likely when a normally cooked or undercooked alkaline-digested pulp is being treated, the fiber is irreparably injured.

In practising the pulp-refinin process utilizing the present invention, I may begin with a normally cooked or even an undercooked pulp, and treat such pulp before alkaline redigestion with a lignin-reactive reagent such as chlorine water, as described in my application for patent. But if the pulp is treated with a lignin-reactive reagent such as hypochlorite solution, or separately with chlorine water and hypochlorite solution, I now add sufiicient free alkali to the hypochlorite solution to maintain it distinctly alkaline throughout the treatment. Under such conditions, degradation of the cellulose content of the pulp is prevented, as chlorine water is quite selective in its action upon lignin and coloring matter only, and alkaline hypochlorite solution acts quite similarly, as the free alkali apparentl represses the intensity of oxidation su ciently to prevent oxidation of the alpha cellulose which is less susceptible to oxidation.

Alkaline redigestion of the pulp may be effected after such treatment in a relatively concentrated alkaline solution at relatively low temperatures, as described in my application for patent, as under such conditions less-resistant celluloses, pentosans, together with reaction products of previous treatment, are readily reacted upon and dissolved from the pulp. The alkalinity of solution employed may be maintained at a point consistent with the product which it is desired to produce. For instance, where the final product is to be suitable both for papermaking and for conversion into cellulose derivatives, the concentration of alkali and temperature conditions should be such as not to efiect mercerization, which im airs papermaking characteristics. When, owever, the final product is to be employed for the manufacture of cellulose derivatives only, it may be preferable to maintain a concentration of alkali and temperature conditions which do efi'ect mercerization, as

under such conditions refinement of the pulp is more rapid and effective, and the mercerized fiber appears for some'purposes to be better suited for conversioninto derivatives. Preferably, where the alkaline tain amount of hypochlorite, as under such conditions reaction upon lignin and other coloring matter in the pulp is promoted.

After alkaline redigestion, the pulp is bleached substantially .to ultimate whiteness, but without physical or chemical injury to the fiber. In accordance with the process of this invention, the redigested pulp is bleached by a series of bleaching treatments, each of which treatments consists in subjecting the pulp to the action of bleach liquor containing relatively small quantities of hypochlorite and sufiicient free alkali to maintain a distinctly alkaline condition during the treatment. If desired, after one treatment, the pulp may be digests ed in a relatively dili'ite alkaline solution at elevated temperature, as this effects a removal of lignin and other non-a1 ha cellulose components which consume ypochlorite, and thus makes possible a reduction of thehypochlorite necessary in reaching the desired whiteness.

The applicability of the process of my invention may best be appreciated by reference to specific examples of procedure, but it is to be understood that this invention is not limited to the precise quantities or proportions of the materials mentioned therein, as variations may be made, depending upon the raw pulp and the product which it is desired to produce.

The raw pulp to be refined may be produced by digesting chipped wood, e. g., spruce under pressure and at elevated temperature in an alkaline liquor such as the usual kraft liquor. In certain instances, it may be de- I sirable to underdigest the chips to produce a high yield of long-fibered, strong pulp, as my refining process is not only applicable toa thoroughly digested pulp, but also to an underdigested pulp; but such latter pulp consumes more chemical in the treatments presently to be described. The pulp is, of course,

washed and screened prior to being subjected to the refining process. The raw pulp may be first treated at a consistency of about 4% to 10% in chlorine water until the chlorine is practically all consumed, the percentage of chlorine necessary depending upon the purity of the pulp. In general, the treatment may be carried out in chlorine water containing 2% to 6% chlorine, based on pulp, for about 30 to 40 minutes. The chlorine may be added to the pulp all at once, or in doses so as to avoid a high chlorine concentration at any time. Where a fairly well-cooked kraft pulp containing about 3% to 5% chlorine is being treated, it is safe to add the chlorine all at once, if the treating temperature is main tained below 75 F. If desired, however, the

chlorine may be added in doses, with intermediate washing of the pulp to remove acid reaction products which may injure the fiber. This treatment does not whiten the pulp, but

tends to deepen its color. In lieu of treating the pulp with chlorine water, particularly when such pulp is well cooked, I may treat it with hypochlorite solution containing sufiicient alkali to maintain such solution distinct ly alkaline throughout the treatment. For instance, the pulp may be treatedwith about 6% to 8% bleach containin about 1% to 2% caustic soda, based on pu p. Or a chlorine treatment followed by an alkaline hypochlorite treatment, or vice versa, preferably with an intermediate washing, may be employed. The chlorinating treatment and/or the alkaline hypochlorite treatment produces reaction upon and removal of lignin and other non-alpha cellulose components of the pulp or renders them susceptible to solution by alkaline digestion.

The pulp is preferably washed substantially free of chemical and reaction products after the first treatment, and is redigested in a relatively concentrated alkaline solution at relatively low temperature. Thus the pulp may be redigested for about one hour at a consistency of about 10% in 5% to 10% caustic soda solution, at a temperature of from 48 to 150 F., under which conditions the resulting fiber remains substantially unmercerized and is hence suitable both for cellulose derivative preparation and for papermaking. If the product is to be employed for cellulose derivative manufacture only, it may be preferable to employ a solution of greater than 10% strength, under such temperature conditions as to produce partial or complete mercerization of the fiber. To this end, redigestion may be carried out at room temperatures or lower, for not only does low temperature favor mercerization, but with a given strength of caustic soda solution apparently effects a removal of pentosans, resins, and ligneous impurities, and accordingly an increase in alpha cellulose content more readily than higher temperature.

In lieu of usingcaustic soda solution for redigestion, I may use the so-called white liquor which is employed for kraft digestion. Such white liquor contains largelysodium hydroxide and sodium sulphide in solution, and usually has a sulphidity of from 20% to 40%. I may also use a liquor containing essentially only sodium sulphide, or one containing, in addition to sodium sulphide and/ or caustic soda, a certain amount of sodium sulphite, which appears to react with the ligneous groups in the pulp. hen the liquor contains chemicals other than sodium hydroxide, it is necessary that the concentration of such chemicals behigher than when sodium hydroxide is used, to effect a given refinement of the pulp, as chemicals such as sodium sulphide or sodium sulphite are not so highly active as sodium hydroxide. Thus, the usual kraft white liquor, to effect the same refinement as a liquor containing about 6% caustic soda alone must contain about 8% caustic soda equivalent.

After alkaline digestion,-

the pulp is washed,

preferably in a washer of the counter-current type, wherefrom unconsumed chemical may be recovered for reuse'in preparing fresh alkaline liquor. Therwashed pulp is-of high alpha cellulose content, and if the papermaking' characteristics of the pulp have been preserved, paper made from such pulp is of much softer texture 'andof much higher tear resistance than that made from the original pulp. Thus, a kraft pulp having a strength of about 150 and a tear resistance of about 250,'after alkalineredh gestion has a strength of about 130 and a tear resistance of about 300 to 320. The drop in strength is doubtless traceable to theremoval of pentosans, but this is not serious when a soft fiber of high which will serve as a rag fiber substitute, is desired. color, however, is usually not much lighter than that of the original pulp. When such pulp is subjected to bleaching,

however, it is found that it bleaches much more readily than the original pulp. Thus, if it were attempted to bleach the original pulpsubstantially to ultimate whiteness, about 40% to 60% bleach, based on fiber,

would be required, and'severe degradation .whiteness by the usual bleaching treatment, its strength falls tov about and its tear m5 resistance to below 150. Soda solubility and copper number tests also indicate degradation of the cellulose content into oxy-celluloses. i

In accordance with my process, of the alkaline-digested fiber substantially to ultimate whiteness is effected without materially degrading the fiber. This is accomplished by subjecting the fiber to a series of alkaline bleaching treatments, with alternating washings preferably with hot alkaline water. Vvashing may be effected in a washer of the countercurrent type, so that the wash water containing the active chemical of one treatment may be employed 120 in a previous bleaching treatment, the last bleaching treatment being effected in fresh bleach liquor. The pulp may also be digested 1n an alkaline solution at elevated temperature between such bleaching treatments.

Thus, the alkaline-redigested pulp may be treated at a consistency of from 6% to 10% in a solution containing 5% to 6% sodium or calcium hypochlorite and 1% caustic soda.

The treatment maybe carried out at temtear resistance, 85

would materially bleaching 110 preferably 115 physical characteristics.

It may be desirable to raise the alkali con tent of the bleachingliquor to 2% or 3%, based on pulp, particularly when the alka line-digested pulp is rather difiicult to bleach. In such case, a higher temperature is maintained during bleachin to increase the rate of bleaching. Prefera l the treatment is commenced at relatively low temperature,

and as the bleach is being consumed the tempera-ture is gradually raised to a al temperature of about 110 F. Or, as previously stated, it may be the pulp at elevated temperature between bleaching treatments. As such alkaline d-igestion removes non-alpha cellulose components from the pulp, it makespossible the use of less bleach in subsequentbleaching treatments, or the elimination of one or more -bleachin treatments. For instance, after ,the first leaching treatment, thepulp may be digested forfabout one hour at about atmospheric boiling temperature in a 2% to '.3% causticsoda solution, then washed before the succeeding bleaching treatment.

After bleaching, the fiber has reached substantiallyultimate whiteness and not only compares favorably inv chemical characterprepared from sulphite pulp, but excels it in 7 ;,-Thus, the white r which I produce bythe process of the present inyention 'is'usuallyystronger and I possesses-higher;tearlkres stance than the at I claim is}:

1.;Afprocess lchfcom rises subjectingcellulose I serieso distinctly alkaline hypochlorite bleaching treatments, one 7 y ,1 or more'bfljsaid{treatments commencing at relat owteniperature and'gradually inreen eatment' to a final much higher alkalinity insaid bleaching tr fiordedby-the resence houtffeach -b caching 2;,1'A com rises subjecting f cellulose distinctly alka. 69 line '11 treatments, one

'orinore y nts icommencingat r roomtem atureor :werandgradually. ma creasin uring-treatm'ent toa final temperae 0 about ,110- F., alkalinity in said chlorite bleach liquor, alkalinit desirable to alkaline-digest containing subjecting such redigested istics with a high grade alpha wood fiber ing treatment.

treatments being afiordedby the.

presence of free alkali throughout each bleaching treatment.

3. A process which comprises partially bleaching cellulose pulp in distinctly alkaline hypochlorite bleach liquor, digesting the par- (0 tially bleached pulp atelevated temperature I in dilute alkaline solution, and treating the digested pulp in a distinctly alkaline hypoin said bleaching treatments being affor ed by the presence of free alkali throughout. each bleaching treatment.

4. A process which comprises redigesting previously digested ulp in alkaline solution and subjecting suc redigested ulp to a series of independent, distinctly al aline hy poc lorite bleaching treatments,alkalinity in said bleaching treatments being aflr'orded b the presence of free alkali throughout each bleaching treatment.

5. A process which comprises treating previously digested raw pulp with a chlorinecontaining lignin-reactive reagent, redigestin the treated pulp in alkaline solution, and su ject'ing such redigested pulp 'to a series oi independent, distinctly alkaline hypochlorite bleaching treatments, alkalinity in said bleaching treatments being afiorded by the presence of free alkali throughout each. bleaching treatment.

6. Aprocess which comprisestreating 'alkaline-digested wood pulp with a chlorinelignin-reactive reagent, redigestr ing the treated pulp in alkaline solution, and ulp to a. series of independent, distinctly al alinehypochlorite bleaching treatments, alkalinity in said bleaching treatments being afforded by the presence of free alkali throughout each 7. A process which comprises treating al- 1 kaline-digested wood pulp with a chlorinecontaining lignin-reactive reagent, redigesting the treated pulp' in alkaline solution, and subjecting the redigested pulp to a series of alternating distinctly. alkaline hypochlorite bleaching treatments and washings, alkalinity "in said bleaching treatments being afforded by the presence of free alkali through each bleaching treatment.

8. A rocess which comprises treating alkalineigested wood pulp with a chlorinecontaining, li'gnimreactive reagent, redigest- I ing the treated pulp in a concentrated alkaline solution at low temperature, and subj ect ing such redigested i pulp to a series of distinctly alkaline. 'hypochlorite bleaching treatments, alkalinit in said bleaching treatments being afiorde by the presence of free alkali throughout each, bleaching treatment.

9. A- process which comprises treating alkaline-digestedwood pulp separately with chlorine watr'and with alkaline hypochlorite liquor,,redigesti'ng the treated pulp in alkaline solution, and subjecting such redicentrated gested pulp to a series of distinctly alkaline hypochlorite bleaching treatments, alkalinit in said bleaching treatments being atl'orded by the presence of free alkali throughout each bleaching treatment.

10. A process which comprises treating alkaline-digested wood pulp with chlorine Water, redigesting the treated pulp in a conalkaline solution at about room temperature, and subjecting-such redigested pulp to a series of distinctly alkaline hypo-- chlorite bleaching treatments, alkalinity in said bleaching treatments being afforded by the presence of free alkali throughout each bleaching treatment.

11. A process which comprises treating alkaline-digested wood pulp with alkaline hypochlorite liquor, redigestin the treated pulp in alkaline solution, and su j ecting such redigested pulp to a series of distinctly alkaline hypochlorite bleaching treatments, alkalinity in said bleaching treatments being afforded by the presence of free alkali throughout each bleaching treatment.

12. A process which comprises treating alkaline-digested wood pulp with a chlorinecontaining, lignin-reactive reagent, redigesting the treated pulp in a solution containing alkali and a much lower proportion of hypochlorite than alkali, and subjecting such redigested pulp to-a series of distinctly alkaline hypochlorite bleaching treatments, alkalinity in said bleaching treatments being afforded by the presence of free alkali throughout each bleaching treatment.

13. A process which comprises treating alkaline-digested wood pulp with a chlorinecontaining, lignin-reactive reagent, redigesting the treated pulp in a caustic soda solution under mercerizing conditions, and subjecting such redigested pulp to a series of distinctly alkaline hypochlorite bleaching treatments, alkalinity in said bleaching treatments being afforded by the presence of free alkali throughout each bleaching treatment.

14. A process which comprises treating alkaline-digested wood pulp with a chlorinecontaining, lignin-reactive reagent, redigesting the treated pulp in a concentrated alkaline solution at low temperature, and subjecting the redigested pulp to a series of alkaline hypochlorite bleaching treatments until the pulp has attained a white color, alkalinity in said bleaching treatments being afforded by the presence of free alkali throughout each bleaching trea ment.

15. A process which comprises treating alkaline-digested wood pulp with a chlorinecontaining, lignin-reactive reagent, redigesting the treated pulp in alkaline solution, partially bleaching the redigested pulp in distinctly alkaline hypochlorite bleach digesting the partially bleached pulp vated temperature in a dilute alkaline solution, and again bleaching the pulp in a dis liquor,

at ele- 1 signature.

GEORGE A. RICHTER.

a substantially I 

